1) Field of the Invention
The present invention relates to a blend of electret fibers and dissimilar non-electret fibers useful for making nonwoven webs. The nonwoven webs are used in making filter fabric and particularly air filter fabric. In particular, the present invention relates to a blend of electret based fibers having one or more charge control agents (CCA) with dissimilar fibers having no CCA. More specifically, the present invention concerns a nonwoven web useful in air filters comprising fibers of polypropylene with one or more charge control agents and polyethylene terephthalate fibers having no CCA.
2) Prior Art
Use of polypropylene electret fibers for filter fabric has been known since the Naval Research Laboratories published Report No. 4364 on May 25, 1954, entitled “Manufacturer of Superfine Organic Fibers” by Van Wente et al. Since that time, many patents have issued on electret fibers and other filter media fabric.
Two types of charges are responsible for the electric fields within the fabric, namely: polarization charges and free (or coulomb) charges. Polarization charges result from the displacement of positive and negative charges within the fiber, e.g., preferential orientation of dipolar bonds. An example of polarization charges is the charge resulting from solidification of the molten fiber in the presence of a strong electric field. The charge polarization produces dipole fields that may be relatively strong at the fiber surface, but decrease rapidly with increased distance from the fiber surface.
Free or coulomb charges generate longer-range forces due to an excess or deficiency of electrons over a substantial length of fiber. These coulomb fields are typically better dust collectors than polarization fields. It is possible to generate “islands” of isolated charges at various locations within the fabric. Additives to the polymer may improve coulomb charge storage in a fiber. These additives are generally known as charge control agents (CCA).
Triboelectric charging generates free charges on the fiber, and results whenever two dissimilar materials are brought into contact, then separated. The amount of charge transfer, and the direction of the electron transfer both depend on the material, their temperature, humidity, etc. Triboelectric charging offers the possibility of a continual source of charge imbalance in fibers, e.g., by mixing fiber materials that have a strong natural tendency to transfer charges to each other. In order for the charge separation to produce useful electric fields, it is important that at least one of the members be a sufficiently good insulator to maintain its charged state for the required period of time (several months to several years).
Knowing the material to be filtered is important in determining the effectiveness of the filter web. For example, common house dust composed of human skin, hair and fur from animals is usually positive in charge. Accordingly, having a filter with areas of negative charge will enhance the filtration of these particles in that opposite charges attract one another. Conversely, if rubber dust is being separated, it generally possesses a negative charge and having areas in the filter that are positively charged is desirable. It is not necessary for the filtered particles to possess any free charge in order for an electret filter to attract them. If the particles can be polarized by the electric field around the fiber, the charge imbalance can generate an attraction. The stronger the electric field, the more effective is the filtration.
U.S. Pat. No. 5,726,107 to Dahringer et al. discloses fabrics made of polymer electret fibers having 0.01 to 30% by weight of at least one charge control agent. This patent lists many classes of CCA compatible with polyolefin polymers.
Blends of electret fibers with non-electret fibers are known and disclosed in U.S. Pat. No. 5,871,845 to Dahringer et al. Column 14, lines 9-50 state that electret yams can be present as mixed yams with other synthetic fibers or natural fibers. However, as explained below, the examples of this patent use similar fibers for both the electret and non-electret fibers.
U.S. patent '845 is specifically directed to polyethylene terephthalate (PET) electret fibers (e-PET) using the same CCA as the present invention. It discloses the use of these e-PET fibers with regular PET fibers. In all examples, 20 percent bicomponent (copolyester/polyester) was employed. In Table 1, Sample 1 was e-PET. Sample 2 was a 50/50 blend of e-PET and PET. Sample 3 was PET alone. The degree of separation T(x) was measured to show the filtration efficiency. From this data it is clear that a 50/50 blend (Sample 2) has a T(x) value midway between e-PET and PET. This indicates that the rule of mixtures holds.
TABLE 1T(x)Sample 1Sample 2Sample 3Particle Sizee-PETe-PET/PETPET0.30.810.710.620.50.850.770.701.00.890.820.77While the above fibers were formed by melt spinning, it is also known to form the fibers by melt blowing.
Lastly, several methods are known to charge electret fibers such as corona charging, triboelectric charging and inductive charging. These methods provide differing relative amounts of free charge and polarization charge.
U.S. Pat. No. 4,874,399 to Reed et al. discloses a melt blown blend of electret fibers comprising poly (4-methyl-1-pentene) and other polyolefin fibers that can accept a filtration-enhancing electrostatic charge and sustain that electret filtration enhancement in the presence of oily aerosols.
U.S. Pat. No. 5,401,466 to Tsai et al. discloses a charging process in which a web containing electrostatic fibers traverses through an electrostatic field and the fibers accept and sustain a filtration enhancing electrostatic charge. Then the web traverses through another charging field where the polarity of the field is reversed. This is called the Tantret™ process.
U.S. Pat. No. 5,558,809 to Groh et al. discloses the use of additives to produce polymer electrets. In particular, this reference discloses the use of a polymer electret, CCA, and an arylsulfide compound and an azochromium complex. Additionally, Groh et al. teach that electret fibers can be charged using a corona or triboelectric process.
To determine filter efficiency, the properties of the polymer, the structure of the filter media, and the determination of the electrostatic charge effectiveness are important considerations. Among these, electrical conductivity is believed to be the most important in affecting the performance of the charged fibers. Subsequently, there is a continual need for improving electret fibers by improving the electrostatic charge effectiveness and particularly by increasing the electrical resistivity of the electret fibers.